Method of error compensation in a coordinate measuring machine

ABSTRACT

A method of calibrating an articulating probe head comprising the steps of measuring an artefact of known dimensions with the workpiece sensing probe mounted on the articulating probe head, in which the articulating probe head is unlocked. An error functional map is generated corresponding to the difference between the measured and known dimensions of the artefact. Subsequent workpieces are measured with the articulating probe head unlocked and the corresponding correction applied. The true dimensions of the artefact may be determined by measuring it with a probe mounted on an articulating probe head in which the axes of the articulating probe head are locked. A mechanical lock is provided to lock the axes of the articulating probe head.

This invention relates to the measurement of the dimensions ofworkpieces using an articulating probe head mounted on coordinatepositioning apparatus. Coordinate positioning apparatus includes forexample coordinate measuring machines (CMM), machine tools, manualcoordinate measuring arms and inspection robots.

It is common practice after workpieces have been produced, to inspectthem on a coordinate measuring machine (CMM) having a quill onto which aprobe is mounted which can be driven in three orthogonal directionsX,Y,Z within a working volume of the machine.

The CMM may be error mapped, for example by using laser interferometers,which enables it to measure a part accurately at slow speeds.

When measuring a workpiece at fast speeds, accelerations of the machinecause dynamic errors. Our previous U.S. Pat. No. 4,991,304 discloses amethod of correcting for these dynamic errors. In this method a firstworkpiece is put on the coordinate measuring machine table and a set ofpoints on the surface of the workpiece are measured at a slow speed toallow accurate readings to be taken. Measurement of the first workpieceis then repeated at a fast speed. The difference between the slow speedreadings and the fast speed readings is calculated and stored. Thestored error value for each measured point takes into account thedynamic deflections of the machine structure at the faster speed.

The next workpiece to be measured is set up on the CMM table andreadings are taken at the fast speed. At this speed the readings areinaccurate but repeatable. Each fast reading is adjusted by adding thecorresponding stored error value and thus compensating for errorsinduced by fast reading. This method has the advantage that a wholeseries of nominally identical workpieces can be measured at fast speedby making a dynamic error map from only one workpiece.

Use of this method allows workpieces to be measured using the CMM at afaster speed but has an upper limit above which it becomesunsatisfactory. This may be due to the CMM becoming inconsistent and/orunstable at high accelerations or the machine being unable to achievethe acceleration demanded.

The limitations described above can be overcome by using a highbandwidth apparatus which is mounted on the coordinate measuringmachine. Such a high bandwidth apparatus is disclosed in U.S. Pat. No.5,189,806 which describes an articulating probe head capable oforientating a probe with two degrees of freedom to enable the probe tobe used in an operation for scanning the surface of workpieces. Ingeneral such a probe head includes two rotary drive mechanisms whichenable a probe to be orientated about two substantially orthogonalrotary axes.

Such an articulating probe head enables fast repeatable scanning.However, use of this articulating probe head has the disadvantage thatit is time consuming to calibrate. Furthermore, the measurement systemof the articulating probe head mounted on a conventional coordinatemeasuring machine is a five-axis system which makes calibration muchmore complicated.

A first aspect of the present invention provides a method of calibratingan articulating probe head, the articulating probe head being mounted onan arm of a coordinate positioning apparatus, in which a surface sensingdevice mounted on the articulating probe head is moved into aposition-sensing relationship with an artefact and a position readingtaken, the method comprising the following steps, in any suitable order:

-   -   a) measuring an artefact whose true measurements have been        determined wherein there is relative movement between the        surface sensing device and the arm of the coordinate positioning        apparatus;    -   b) generating an error function or map corresponding to the        difference between the measurements obtained in step a and the        true measurement of the artefact;    -   c) measuring subsequent workpieces wherein there is relative        movement between the surface sensing device and the arm of the        coordinate measuring apparatus; and    -   d) correcting measurements of subsequent workpieces obtained in        step c using the error function or map generated in step b.

The true measurements of the artefact may be determined by measuring theartefact, wherein there is no relative movement between the surfacesensing device and the arm of the coordinate positioning apparatus.Preferably this is done at a slow speed to remove dynamic errors.

The true measurement of the artefact may be determined by using acalibrated artefact.

Preferably the calibrated artefact comprises at least one circularprofile.

The surface sensing device may comprise for example a workpiece sensingprobe or a stylus.

The artefact may comprise a workpiece in the series of workpieces.Alternatively the artefact may have features the size and location ofwhich approximate the workpiece. The artefact may have the same surfacefinish as the workpiece or alternatively may mimic the surface finish ofthe workpiece.

In step a) the arm of the coordinate measuring apparatus may bestationary. Alternatively the arm of the coordinate measuring apparatusmay be moving at a constant velocity. This eliminates dynamic forcesfrom the coordinate measuring apparatus.

The measurements taken may be discrete measurements (i.e. using a touchtrigger probe) or continuous measurements (i.e. using a scanning probe).

The surface sensing device may be a contact probe, such an analogue(scanning) probe or a touch trigger probe. Alternatively the surfacesensing device may be a non-contact probe, such as a capacitance,inductive or optical probe.

A second aspect of the invention provides an articulating probe headcomprising a first mount for mounting on a support and a second mountonto which a surface sensing device may be mounted, the second mountbeing rotatable relative to the first mount about one or. more axis,characterized in that the articulated probe head is provided with atleast one mechanical brake to lock the position of the second mountrelative to the first mount about at least one axis; and whereby atleast one position measuring device is provided to determine theposition of the second mount relative to the first mount about said atleast one axis.

Preferably the articulating probe head includes a rotary memberrotatable about at least one axis, and wherein the lock has a lockmember which moves between the first and second positions, in its firstposition it engages with the rotary member to lock the rotary member inposition and in its second position it disengages the rotary member,allowing the rotary member to rotate.

The rotary member may comprise a drive belt or a driven wheel.

The rotary member may be provided with a tooth profile and wherein atoothed lock assembly is provided on the lock, such that in the firstposition the teeth of the lock assembly and the rotary member interlock.

The lock may be provided with an actuator to move the lock memberbetween its first and second positions.

The point of contact between the actuator and the lock member may beseparated transversely from the point of contact between the lock memberand the rotary member. The lock member may comprise a lever assemblypivotable about a pivot point and wherein the point of contact betweenthe lock member and the rotary member is located between the pivot jointand the point of contact between the actuator and the lock member. Thelock member may comprise a lever assembly. Biasing means may be providedto bias the lock member against the rotary member when the lock memberis in its first position.

Preferred embodiments of the invention will now be described by way ofexample with reference to the accompanying drawings wherein:

FIG. 1 is a perspective view of a coordinate measuring machine (CMM)provided with an articulating probe head;

FIG. 2 illustrates a cross-sectional view of the articulating probehead;

FIG. 3 is a flow chart of the method;

FIG. 4 illustrates a bore being measured with the articulating probehead locked;

FIG. 5 illustrates a bore being measured with the articulating probehead unlocked;

FIG. 6 illustrates a mechanical brake in its upper position;

FIG. 7 illustrates the mechanical brake of FIG. 6 in its lower position;

FIG. 8 is a perspective view of a first embodiment of a calibrationartefact;

FIG. 9 is a cross-section of the calibration artefact of FIG. 8;

FIG. 10 is a side view of a second embodiment of a calibration artefact;

FIGS. 11 and 12 are side views of a second embodiment of a mechanicalbrake in its locked and unlocked positions respectively;

FIGS. 13 and 14 are side views of a third embodiment of a mechanicalbrake in its locked and unlocked positions respectively;

FIGS. 15 and 16 are side views of a fourth embodiment of a mechanicalbrake in its locked and unlocked positions respectively;

FIG. 17 is a side view of a fifth embodiment of the mechanical brake inits locked position; and

FIG. 18 is a side view of an articulated probe head with three axes ofrotation.

FIG. 1 illustrates an articulating probe head mounted on a coordinatemeasuring machine. The coordinate measuring machine 10 comprises a table12 on which a workpiece may be placed and an arm 14 movable in X,Y and Zrelative to the table 12. An articulating probe head 16 is mounted onthe arm 14 of the CMM. The articulating probe head 16 allows aworkpiece-measuring probe 18 mounted on it to be rotated about twosubstantially orthogonal axes A1 and A2.

The machine arm 14 may therefore be moved in X,Y and Z directions underthe action of X,Y and Z drives (not shown) of the coordinate measuringmachine. X,Y and Z scales (not shown) show the instantaneous coordinatesof the position of the arm 14. Rotary drive means in the articulatedprobe head (not shown) enable movement of the probe about the two rotaryaxes A1 and A2. This movement is measured by rotary scales (not shown)inside the articulating probe head 16. Signals from the probe 18indicating the deflection of the probe stylus are combined with thereadings from the X,Y and Z scales of the CMM and the rotary scales ofthe articulating probe head to calculate the position of the stylus tipand thus the surface of the workpiece.

As illustrated in FIG. 2, the articulating probe head 16 comprises afixed part formed by a base or housing 20 supporting a movable part inthe form of a shaft 22 rotatable by a motor M1 relative to the housing20 about an axis A1. The shaft 22 is secured to a further housing 24which in turn supports a shaft 26 rotatable by motor M2 relative to thehousing 24 about an axis A2 perpendicular to the axis A1.

A probe 18 with a stylus 28 having a workpiece-contacting tip 30 ismounted onto the articulating probe head 16. The arrangement is suchthat the motors M1,M2 of the head can position the workpiece-contactingtip 30 angularly about the axes A1 or A2 and the motors of the CMM (notshown) can position the articulating probe head 16 linearly anywherewithin the three-dimensional coordinate framework of the CMM to bringthe stylus tip 30 into a predetermined relationship with the surfacebeing scanned.

Linear position transducers (not shown) are provided on the CMM formeasuring linear displacement of the articulating probe head 16 andangular position transducers T1 and T2 are produced in the articulatingprobe head 16 for measuring angular displacement of the stylus 38 aboutthe respective axes A1 and A2.

Referring to FIG. 3 the following procedure is used in the presentinspection method. A workpiece from the series of workpieces to bemeasured is placed on the table 12 of the coordinate measuring machine40. Alternatively an artefact could be used which approximates theworkpiece, in particular having features the size and/or location ofwhich match the features of the workpiece.

The rotational axes of the articulating probe head are locked, or heldstationary such that the probe 18 is not able to move about the rotationaxes A1 and A2. Thus the system is effectively a probe mounted on acoordinate measuring machine. With the articulating probe head solocked, the workpiece or artefact is scanned or measured 42.

FIG. 4 illustrates a bore 56 being scanned with the articulating probehead 16 locked. In this case the arm 14 of the CMM must move as shown byarrow A to allow the workpiece-contacting tip 30 of the probe 18 tomeasure the internal surface of the bore.

In the next step of the method, the articulating probe head 16 isunlocked so that the probe 18 may move about the rotary axes A1 and A2.The workpiece is then scanned or measured with the articulated probehead unlocked 44.

FIG. 5 illustrates a bore 56 being scanned with the articulating probe16 unlocked. The machine arm 14 may be positioned such that it isaligned with the axis of symmetry 57 of the bore 56 and held stationarywhilst the probe 18 is moved about the rotational axes of thearticulating probe head 16. Alternatively the arm of the CMM may move atconstant velocity along the axis of symmetry 57 of the bore 56 as theprobe 18 is rotated about the rotation axes of the articulating probehead 18. In this case the internal surface of the bore is scanned in aspiral profile. In both cases i.e. when the arm 14 is stationary ormoved at constant velocity, no dynamic forces are applied to themachine.

In a next step the measurement data obtained during the scan with thearticulating probe head locked is compared with the measurement dataobtained from the scan with the articulating probe head unlocked. Thisis used to generate an error function or map 46. This error function ormap allows the errors caused by the articulating probe head at eachpoint on the surface of the workpiece to be determined.

Subsequent workpieces in the series of workpieces are set up on thecoordinate measuring machine 48. Preferably automatic means (not shown)place each of the succession of substantially identical workpieces for aproduction run in at least nominally the same position and orientationon the CMM table. One of the subsequent workpieces is scanned with thearticulating probe head unlocked. The measurement data obtained duringthis scan is corrected using the error function or map 52 createdpreviously.

For best results, substantially the same measurement path is used formeasuring the subsequent workpiece (50, FIG. 3) as for the initialmeasurement with the probe head unlocked (44, FIG. 3).

This method takes advantage of the accuracy of the CMM and therepeatability of the articulating probe head to enable fast and accuratemeasurements of workpieces to be taken without the requirement forcalibrating the articulating probe head.

This method corrects for geometric errors in the articulating head. Itis possible to use this method to also correct for dynamic errors whichmay for example be caused by bending in the articulated head or twistingof the quill of the CMM. To correct for dynamic errors, the workpiece isscanned at a slow speed in step 42 to obtain measurement data with nodynamic errors. The workpiece is then scanned at a fast speed in step 44so that the error map or function generated in step 46 includes bothgeometric errors and the dynamic errors caused by scanning at a fastspeed. The subsequent measurements in step 50 are measured at a fastspeed. The dynamic errors created during this scan are corrected by theerror map or function in step 52.

However, if the articulating head has a good mechanical design, therewill be negligible dynamic errors and thus the workpieces can bemeasured at any speed in each case.

Although the above description is directed at the use of scanningprobes, it is also suitable for taking measurements with a touch triggerprobe, in which discrete measurements are taken at points on the surfaceof the workpiece. Furthermore, the method is also suitable for use witha non-contact probe, for example a capacitive, inductive or opticalprobe. The rotational axes of the articulating probe head may be lockedby various means. For example, the articulated probe head may be heldstationary on the servo motors, or a separate locking device may beused.

FIGS. 6 and 7 illustrate a mechanical brake used to lock the rotationalaxes of the articulating probe head. The brake 60 comprises a brake pad84 which may be pushed onto a pulley belt 86 of a driven wheel tothereby lock movement. The brake 84 may be made of rubber, which has ahigh coefficient of friction. The brake pad is pushed against the pulleybelt by a lever mechanism which will be described in more detail below.A pin 62 is movable between upper and lower positions by the action of aswitching solenoid. FIG. 6 shows the pin 62 in its upper position andFIG. 7 shows the pin 62 in its lower position. The lower end of the pinis connected to first and second arms 66,68 by a pivot 64. The first arm66 is connected to the pin 62 at one end by the pivot 64 and to a thirdarm 74 at its second end by another pivot 70. The third arm 74 isprovided with a pivot 78 at one end, about which it may rotate relativeto a fixed surface 88.

The second arm 68 is connected to the pin 62 at one end by the pivot 62and to a fourth arm 76 at its second end by another pivot 72. The fourtharm 76 is attached to a fixed surface 82 by a pivot 80 at its other end.The fourth arm is provided with a brake pad 84 on a surface adjacent thepulley belt 86.

FIG. 6 shows the pin 62 in its upper position with the brake pad 84 inits disengaged position. A current may be passed through the switchingsolenoid to push the pin 62 into its lower position shown in FIG. 7.This downward movement of pin 62 and its pivot 64 causes first andsecond arms 66,68 to swivel about pivot 64 and move outwards becomingcloser to a horizontal position. This movement of the first and secondarms 66,68 causes third and fourth arms 74,76 to rotate about theirrespective pivots 78,80 so that the ends adjacent the first and secondarms 66,68 are pushed away from the pin 62. The brake pad 84 is therebypushed against the pulley belt 86 to act as a brake. Screw 90 located inthe fixed block 88 acts as a stop to define the maximum movement of thethird arm 74 and thus also the fourth arm 76.

FIGS. 11 and 12 illustrate a second embodiment of the mechanical brake.FIG. 11 illustrates a drive wheel 144 which drives a driven wheel 142via a drive belt 140. The brake is a pinch brake in which a brake shoeis pushed against the drive belt in the locked position and held awayfrom the drive belt in the unlocked position. The drive shoe 149 rotatesbetween its locked position illustrated in FIG. 11 and its unlockedposition illustrated in FIG. 12 about a pivot 150. The pivot 150includes adjustment means such as a cam which enables fine adjustment ofthe position of the brake shoe 149.

The position of the brake shoe is actuated by a solenoid 146. In thelocked position illustrated in FIG. 11 the solenoid 146 pushes a pin 148against the brake shoe 149, causing it to rotate about the pivot 150into its locked position. In the unlocked position the solenoid 146pushes the pin 148 upwards out of contact with the brake shoe 149 and areturn spring 152 biases the brake shoe into its unlocked position.

FIGS. 13 and 14 illustrate a third embodiment of the invention. Theembodiment is similar to that shown in FIGS. 11 and 12 and similarfeatures use the same reference marks.

A lever assembly 154 is provided which extends from a fixed surface 158to lie adjacent the drive wheel 140. A flexure 156 allows pivoting ofthe lever assembly and biases the lever away from the drive belt.

When the brake is in its locked position as illustrated in FIG. 13, thebrake shoe 149 pushes against the lever 154 which in turn pressesagainst the drive belt 140. When the brake is in its unlocked position,the brake shoe 149 is biased towards its rest position by a returnspring 152 and the lever assembly 154 is biased to its rest position bythe flexure 156. The lever assembly acts as a separating member andseparates the point of contact between the lever and the drive belt fromthe point of contact between the brake shoe and the lever. The leverassembly prevents the brake from overlooking or jamming in eitherdirection of rotation of the driven member, which may be caused byvariation in the thickness of the drive belt and the roundness of thedriven wheel.

A weakened section 155 of the lever assembly 154 enables it to act as aspring lever. Thus when the brake shoe 149 creates a force F against thelever assembly, the lever is able to bend over length L and therebyaccommodate any error in the thickness of the drive belt or roundness ofthe driven wheel.

The brake systems described in FIGS. 6 and 7 and 11-14 arenon-incremental and thus allow the driven wheel to be locked in anyposition.

FIGS. 15 and 16 illustrate a fourth embodiment of the mechanical brake.This embodiment has some components in common with FIGS. 13 and 14 andsimilar features use the same reference numerals.

In FIGS. 15 and 16, the brake acts on the drive wheel 142 rather than adrive belt, which is located on one side of the drive wheel. The drivenwheel 142 is provided with a tooth surface 160 on its circumference.This may be provided for example by a ring with an outer surface havinga tooth profile. A complementary tooth assembly 162 is provided on thelever assembly 154. Thus in the locked position illustrated in FIG. 15the teeth in the toothed surface 160 of the driven wheel and the toothassembly 162 of the-lever engage to lock the drive wheel 142 inposition. As this embodiment has interlocking teeth, the brake will holdthe drive wheel in incremental positions, for example of 1°. Thisembodiment has the advantage that it is effective even when there areerrors in the thickness of the belt and/or the roundness of the wheel.Once the teeth have engaged, the flexibility of the lever assemblyenables the brake shoe to exert more force F to bias the lever into thebrake position, ensuring the brake remains on.

In any of these embodiments, a rotary encoder system may also beprovided, for example it may be located on the drive wheel. Asillustrated in FIGS. 13 and 14, this may comprise a rotary scale ringmounted on the driven wheel and a readhead located on a relatively fixedsurface adjacent the wheel. Any slight movement in the brake positioncan be read by the encoder and a correction applied to the measurementdata.

In a further embodiment of the brake system, the brake may be used tohold the articulated probe head in a repeatable position. FIG. 17illustrates the a brake pad 184 pivotally mounted on a mount 182. Themount in turn is mounted on a piezo stack 188. As in previouslyembodiments, the articulated probe is locked by pressing the brake padagainst a rotary part such as the driven wheel 186. The position of thearticulated probe head is determined by reading the position from theencoder 190. If the articulated probe head is not in its desiredposition, this embodiment enables the position to be adjusted. The brakepad 184 remains in contact with the rotary part 186 whilst a voltage isapplied to the piezo stack 188 to adjust its height h. This has theeffect of moving the position of the brake pad 184 and thereby rotatingthe driven wheel 186 which is in contact with the brake pad. Theposition of the driven wheel 186 may therefore be adjusted until theoutput from the encoders 190 gives the desired position. The piezo stack188 has the advantage that it does not dissipate much heat. Furthermoreit produces a high force with small movements (a few hundred microns)which enables fine adjustment. Other actuators may be used, for examplea hydraulic ram.

This mechanical brake is suitable for any type of articulating probehead which has one part rotating relative to another. For example thebrake is suitable for the probe head disclosed in U.S. Pat. No. RE35510,in which the articulated probe head moves between a plurality of indexedangular positions.

Where the articulating probe head provides rotation about two or threeaxes, a mechanical brake may be provided for each axis.

The surface sensing device could comprise for example a surface sensingprobe, a stylus or a camera.

FIG. 18 illustrates an articulating probe head 18 onto which a camera198 is mounted. The camera 198 is rotatable about three axes 192, 194,196.

Although the above embodiments describe a brake acting against a drivebelt or a driven wheel, the brake may act on any rotary part, such as ashaft or motor pinion.

In all of the above embodiments, the mechanical lock uses friction tolock the articulating head. Although in the above embodiments the lockis actuated by a solenoid, other means may be used, such as hydraulic,pneumatic, motor, piezo or gravity. The mechanical lock is actuated intoits engaged and disengaged positions but once in position no power isrequired.

Other types of brake, such as a disc brake may also be used.

It is also possible to have a brake in which there is no contact betweenthe brake member and a rotary part. A magnetic brake may comprise one ormore electromagnets in close proximity to a ferrous rotary part. Thebrake is actuated by turning on the electromagnets which will preventthe rotary part from rotating.

Use of a mechanical lock to lock the axes of the articulating probe headhas several advantages over sensing the motors of the articulating probehead to hold it in position. With the use of a mechanical lock, themotors of the articulating probe head are allowed to rest, thus reducingthe temperature of the system. The reduced thermal effect improves themetrology of the system. Furthermore, the mechanical lock provides afixed system, compared to serving the motors, thus improving themetrology of the system.

In a second embodiment of the invention, the articulated probe head iscalibrated by scanning a calibrated artefact.

FIG. 8 illustrates an artefact 100 which comprises circular profiles102,104,106 of different diameters. These circular profiles 102,104,106are calibrated, for example by form measuring apparatus, and are thus ofknown dimensions.

The circular profiles have a centre line 110, and this centre line maybe oriented with a desired direction, for example, the machine X,Y and Zaxes, by means of first and second indexing devices 112,114.

The calibrated artefact 100 is orientated with its centre line 110aligned with a first axis, for example the machine's X-axis. The machinearm moves the articulating probe head and probe into a position alignedwith this axis such that the circular profile may be scanned by theprobe by rotation about axes A1 and A2 of the articulated probe head,whilst the machine arm remains stationary.

The circular profiles 102,104,106 are preferably scanned at a fastspeed, i.e. the speed to be used for subsequent measurement.

The measurements of the circular profile are compared with the knownforms of the circular profiles to create an error map relating to thedirections and speed of the measurement. The calibrated artefact 100 isthen oriented to align its centre line 110 with a new direction, usingthe indexing devices, and the method is repeated to create a new errormap relating to this new direction.

Once the calibrated artefact 100 has been measured at differentorientation, for example 7.5° increments, the data may be interpolatedto derive the error data for the positions of the calibrated artefact inbetween these orientations. Likewise, the data relating to the circularprofiles of different diameters may be interpolated to create error datafor circular profiles having diameters in between the measured values.The interpolation may comprise techniques such as linear or polynomialbest fits.

By measuring the calibrated artefact at a fast speed, the error mapcorrects for dynamic errors. However as discussed earlier, this may notbe necessary if the articulated head has a good mechanical design.

Subsequent workpieces measured using the probe mounted on thearticulating probe head are corrected using the error map relating tothe relevant direction.

FIG. 10 shows an alternative type of calibrating artefact 120 in whichseveral cones 130,132,134 are provided with circular profiles122,124,126 of different diameters which are aligned along differentmachine axes, thus removing the requirement of an indexing device.

The above description describes an articulated probe head mounted on aCMM. However this invention is suitable for any type of coordinatepositioning apparatus that has one or more axis of movement. For examplethe articulated probe head may be mounted on a single axis system.

1. A method of calibrating an articulating probe head, the articulatingprobe head being mounted on an arm of a coordinate positioningapparatus, in which a surface sensing device mounted on the articulatingprobe head is moved into a position-sensing relationship with anartefact and a position reading taken, the method comprising:determining true measurements of an artefact by measuring the artefactwith no relative movement between the surface sensing device and the armof the coordinate positioning apparatus; measuring the artefact whosetrue measurements have been determined, wherein at least one motor ofthe articulating probe head is driven to provide relative movementbetween the surface sensing device and the arm of the coordinatepositioning apparatus; generating an error function or map correspondingto the difference between the measured artefact and the truemeasurements of the artefact; measuring subsequent workpieces wherein atleast one motor of the articulating probe head is driven to providerelative movement between the surface sensing device and the arm of thecoordinate measuring apparatus; and correcting subsequent workpieces byusing the generated error function or map.
 2. A method according toclaim 1 wherein the step of measuring the artefact when there is norelative movement between the surface sensing device and the arm of thecoordinate positioning apparatus is carried out at a slow speed.
 3. Amethod according to claim 1 wherein the artefact comprises a workpiecein the series of workpieces.
 4. A method according to claim 1 whereinthe artefact has features the size and location of which approximate aworkpiece.
 5. A method according to claim 4 wherein where the featuresof the artefact differ from the features of the workpiece, thecorrection of measurements of subsequent pieces is achieved byinterpolation.
 6. A method according to claim 1 wherein the arm of thecoordinate measuring apparatus is stationary.
 7. A method according toclaim 1 wherein the arm of the coordinate measuring apparatus is movingat a constant velocity.
 8. A method according to claim 1 wherein themeasurements comprise continuous measurements.
 9. A method according toclaim 1 wherein the measurements comprise discrete measurements.
 10. Amethod according to claim 1 wherein the surface sensing device is acontact probe.
 11. A method according to claim 1 wherein the surfacesensing device is a non-contact probe.
 12. A method according to claim 1wherein a locking mechanism prevents relative movement between thesurface sensing device and the arm of the coordinate positioningapparatus.
 13. A method according to claim 1 wherein the articulatingprobe head is provided with at least one motor to rotate the surfacesensing device relative to the arm of the coordinate positioningapparatus, and wherein said at least one motor is used to preventrelative movement between the surface sensing device and the arm of thecoordinate positioning apparatus.
 14. A method according to claim 1wherein measuring an artefact and measuring subsequent workpieces arecarried out at a fast speed.
 15. A method according to claim 1 whereinmeasurement paths used in measuring subsequent workpieces and indetermining the true measurements of the artefact are substantially thesame.
 16. A method of calibrating an articulating probe head, thearticulating probe head being mounted to a quill of a coordinatepositioning apparatus, in which a surface sensing device mounted on thearticulating probe head is moved into a position-sensing relationshipwith an artefact and a position reading taken, wherein the quill of thecoordinate positioning apparatus can be driven along three linear X, Yand Z axes under the action of X, Y and Z drives, the method comprising:determining true measurements of an artefact; measuring the artefactwhose true measurements have been determined, wherein at least one motorof the articulating probe head is driven to provide relative movementbetween the surface sensing device and the quill of the coordinatepositioning apparatus; generating an error function or map correspondingto the difference between the measured artefact and the truemeasurements of the artefact; measuring subsequent workpieces wherein atleast one motor of the articulating probe head is driven to providerelative movement between the surface sensing device and the quill ofthe coordinate positioning apparatus; and correcting the measurements ofthe subsequent workpieces using the generated error function or map. 17.A method according to claim 16, wherein the true measurements of theartefact are determined by measuring the artefact with no relativemovement between the surface sensing device and the quill of thecoordinate positioning apparatus.
 18. A method of calibrating anarticulating probe head, comprising the steps of: taking an articulatingprobe head, a surface sensing device, and a coordinate positioningapparatus having a moveable arm, wherein the surface sensing device isattached to the moveable arm via the articulating probe head, whereinthe articulating probe head comprises at least one motor for providingrelative movement between the surface sensing device and the moveablearm; moving the surface sensing device into a position-sensingrelationship with an artefact and taking a first set of measurementdata, wherein the first set of measurement data is taken with thearticulating probe head configured such that there is no movement of thesurface sensing device relative to the moveable arm of the coordinatepositioning apparatus; moving the surface sensing device into aposition-sensing relationship with the artefact and taking a second setof measurement data, wherein the second set of measurement data is takenwith the articulating probe head configured such that there is movementof the surface sensing device relative to the moveable arm of thecoordinate positioning apparatus; and comparing the first set ofmeasurement data with the second set of measurement data and generatingan error map or function describing the measurement errors caused by thearticulating probe head.